The invention relates generally to television receivers and monitors and, more particularly, to focus flutter prevention in TV receivers and monitors.
As used herein, the terms cathode ray tube (CRT), picture tube, and kine have the same meaning and are used interchangeably. Circuit elements repeated in different drawing figures are referenced by the same numerals.
Kine driver circuits in TV receivers or monitors are often prone to a problem referred to herein as focus flutter. Focus flutter occurs shortly after a television receiver is turned on. When power is first applied to a television receiver or monitor, the CRT filament is cold and there is no emission from the cathodes and, therefore, no beam. This lack of beam current causes the CRT cathodes to appear as open circuits. With no beam current the beam current limiter does not reduce contrast which controls the video gain of the TV receiver. At a maximum contrast control setting the video output signal from the source of video signals, e.g., a luma/chroma integrated circuit, is amplified to the greatest extent, which causes the high gain kine drivers to saturate.
In normal operation, a CRT""s cathode potential is about 50-150 volts higher than its control grid (G1, see FIG. 1) potential. As a result, beam current is held within a normal range. However, the above-described condition of kine driver saturation causes the cathode voltage to fall to a level at or close to the level of the voltage on the grid G1 to produce a zero bias condition. In this zero or near zero bias condition, when the CRT filament has heated the cathodes enough to have emission, excessive beam current will begin to flow.
With some picture tubes excessive beam current can be partially intercepted by the focus electrode (F, see FIG. 1), causing focus current to flow. This, in turn, causes the focus voltage to drop and defocuses the beam, causing even more beam current to strike the focus electrode. This positive feedback phenomenon manifests itself as a brightness fluctuation of the picture, known to skilled artisans as focus flutter.
The focus flutter problem occurs, typically, for a few seconds after a short 5-8 seconds warm-up delay, during a turn-on with a high IRE luminance signal when the brightness and/or contrast controls are set high enough. The problem is relieved after 1 to 2 seconds because as the beam current begins to flow the beam limiter reduces the video gain and the kine drivers come out of saturation and the zero bias condition disappears. Factors contributing to focus flutter include the geometry of the electron gun assembly of the CRT, the underheated cathode, excessively high drive of the cathode before the beam limiter activates, and less than completely stabilized and regulated power supplies.
The most commonly used approach to reduce the problem of focus flutter has been to lower the maximum drive voltage applied to the cathode of the picture tube by reducing, for example, the signal applied to the kine driver""s input or by reducing the kine driver""s gain. A disadvantage of such an approach is the loss of the light output of the receiver at normal operating conditions. Another approach has been to blank the screen during the warm up period of the receiver through software means. This requires dedication of some memory resources that can be at a premium. The most fundamental approach is to redesign the picture tube guns by increasing the aperture of the central hole in the control grid G1 and the Focus electrodes to reduce the beam current interception. However, redesigning the picture tube is undesirable as it adds to the cost, time and manpower constraints and can degrade the beam spot size, i.e., resolution, at normal operating conditions.
Another solution proposed utilizes a diode clamp in the kine driver circuit to clamp voltage at the grid GI to a voltage level lower than the cathode voltage level, that prevents excessive beam current which would otherwise cause focus flutter, immediately following a cold cathode turn on. This solution has several drawbacks in that it alters operation of the receiver during both turn-on and normal operating conditions, during kine driver amplifier saturation the bias cannot go as low as 4V, whereas existing picture tubes can run into focus flutter mode during a turn-on at a bias voltage as high as 20V. The addition of diodes at the collectors of the kine driver amplifiers reduces the amplifiers"" bandwidth by increasing their collectors"" capacitance, which in turn reduces the resolution of the TV receiver.
An inventive method for preventing focus flutter display in a TV receiver or monitor includes the steps of amplifying a received signal for driving cathode elements of a cathode ray tube; and delaying initial full amplification of the signal during the amplifying step for a duration sufficient to prevent focus flutter display on the tube.
An inventive kine driver circuit that prevents display of focus flutter includes an amplifier for amplifying and coupling received video signals to cathode elements of a picture tube; and a control circuit for delaying coupling of amplified the video signals to the cathode elements for a duration sufficient to prevent focus flutter display on the tube.